LEDs are being used increasingly more and in various applications. LEDs find their ways into the backlighting of LCDs, into traffic lights and traffic signs, automobiles and domestic illumination, etc. The light output of an LED directly depends on the current flowing through the LED. A current control circuit is therefore used to regulate the current flow through the LEDs, preferably so as to maintain a constant current during all operating conditions.
Light-emitting diodes (LEDs) are driven by a specific driver circuit (driver). Typically one such driver can control one group that forms one segment of LEDs that are connected to the driver. If two or more segments (multiple groups of LEDs, each group having for example a different location or a different color) need to be driven, multiple drivers can be used or extra switches can be used in series with, or parallel to, the LEDs. Using multiple drivers is not preferred because of higher costs and larger bill-of-materials. A LED driver behaves as a current source, i.e., it has a high output-impedance. As a result, series switches are not preferred because in this way either the complete string is disconnected or parallel branches are disconnected. This gives the problem of LED impedance matching and the driver needs to switch simultaneously with the series switch to a new amplitude setting. Consequently the cost-effective choice for the extra switch is putting the switch in parallel to a portion of the LED string. Such a parallel switch is referred to as a “bypass LED dim switch” or bypass-switch. Accordingly, bypass-switches are in principle a good choice for increasing the level of segmentation without using a large number of drivers. One driver can be used to drive multiple segments.
However, problems may occur regarding the control of the bypass-switch. The bypass-switch needs to be reliably controlled by a stable pulse width modulated (PWM) control signal at a phase required by the system. This stable PWM signal ensures the required brightness setting and required color stability in case of, e.g., RGB LED systems. The bypass-switch needs to operate in an environment where large common mode variations occur because of bypass actions from other bypass-switches used in the LED string. These other bypass-switches have in principle their own, individually programmed and independent PWM control signal and phase. As a result, the challenge in operating the bypass-switch is in providing stable reliable operation in an electrical environment that experiences large common mode variations.